1. Field of the Invention
The invention relates to the field of mass storage devices.
2. Background Art
In a typical rotating media mass storage system,data is stored on magnetic or magneto optic disks in a series of concentric "tracks." These tracks are accessed by a read/write head which detects variations in a magnetic orientation of the disk's surface. These variations represent information stored on the disk surface.
The tracks are electronically divided into a plurality of smaller fields, or physical records, or "sectors." Because storage disks are used as random access memory in many applications, such as personal computers, related information is not always written consecutively in the individual tracks. In addition, as old data is removed and new data added, it is not always possible to write new data in adjacent sectors or even adjacent tracks. Therefore, it is important for the disk drive to be able to quickly and accurately locate individual sectors of a track.
One measure of disk drive performance is "rotational latency," the average time it takes a data head to access a given sector, once the head is positioned on a track. In disk drives, rotational latency is defined as one-half of a disk rotation since, on the average, a desired record is 180 degrees from the initial position of the data head. It is desirable to minimize rotational latency.
The reference point for an angular position of the head with respect to the particular track being accessed is provided by an "index" which defines a starting location for each track. The index point is typically determined by decoding fiducial data written as part of the servo information used for control of data head positioning. Before data in a sector can be accessed, an electronic signal must be produced to indicate that the sector sought is under the read/write head. The production of this signal can be accomplished in two ways.
One prior art solution to the problem of sector location is to provide a single sector counter which continuously monitors sector position. The sector counter method can be utilized in a fixed sector-length format, where each physical record on the disk is of equal length. In this scheme, a counter is periodically initialized by the index mark, and is stepped by a clock which is synchronized in phase and frequency with the disk rotation rate. This clock and the index mark are conventionally generated in the phase-lock oscillator (PLO) of the servo system. Each increment of the counter defines a new sector. This is referred to as a "hard" sector counter scheme. FIG. 6 illustrates this prior art.
A disadvantage of direct application of the prior art hard sector counter occurs when the sector length is variable from track to track, such as in a constant density recording scheme. In constant density recording schemes, the disk is divided into a plurality of concentric "zones." The write frequency is different for each zone so that the bit/inch density is approximately constant for the entire disk. When the write frequency is highest, there are a greater number of sectors per track. When the read/write head is moved from zone to zone, the sector counter must be reset for the new zone parameters by waiting for the occurrence of the index point. Hence, the latency time increases when the prior art sector counter scheme is utilized in a constant density recording scheme. Thus, application of the prior art hard sector method is not suitable for use in a practicable, constant density recording scheme.
A second prior art method to provide accurate sector location information is known as a "soft" sector scheme. In a soft sector method, sector boundaries are identified by means of "address marks" formed as part of the sector. The address mark is such that it will not be mistaken for data, such as a void of 3 bytes, or a violation of run-length properties of the chosen recording code. The soft sector method has the well-known disadvantage of susceptibility to noise and media defects. In addition, the soft sector scheme takes up disk space otherwise available for data recording. The noise problem affects the reliability of the soft sector scheme. A soft sector scheme requires no sector counter; sector identification is provided by detection of an address mark by special hardware provided for that purpose.
Because of the disadvantages of the soft sector scheme, it is desirable to utilize a hard sectoring scheme in a zoned-density recording format. However, there is a time penalty when using the prior art sector counter method since it requires revalidation at index of the sector count when moving from zone to zone. One solution is to provide a separate dedicated prior art sector counter for each zone and to switch to the appropriate counter when changing zones. However, this adds to the complexity and cost of the hardware implementation, and is impractical when a large number of zones is required.
Therefore, it is an object of the present invention to provide an economical hard sector counting scheme applicable to zoned-density format.
It is another object of the present invention to provide a sector counting scheme which may be implemented using a single sector counter.
It is yet another object of the present invention to provide a sector counter for use in zoned-density recording system which does not increase latency.